(Z)-2-PHENYL-4-BENZYLIDENE-5(4H)-OXAZOLONES AND (E)-2-PHENYL-4-BENZYLIDENE-5(4H)-OXAZOLONES AS DIENOPHILES - IMPROVED SELECTIVITY BY THE USE OF HETEROGENEOUS CATALYSTS

Silica gel treated with AlEt(2)Cl or TiCl4 efficiently promotes the re action of cyclopentadiene with (Z)-2-phenyl-4-benzylidene-5(4H)-oxazol one (1a) with very Little Z/E isomerization. Silica gel treated with T iCl4 and ZnCl2 supported on silica gel leads to very high yields for t he reaction of cyclopentadiene with (E)-2-phenyl-4-benzylidene-5(4H) - oxazolone (1b). Furthermore, E/Z isomerization is almost suppressed, w hich is not achieved with homogeneous Lewis acids. Whereas for the rea ction of 1a with cyclopentadiene the exo cycloadduct (3a) is always th e major product, for the reaction of cyclopentadiene wth 1b the endo(2 b)/exo(3b) selectivity is reversed as a function of the catalyst. Thus , in the reactions catalyzed by silica gel, alumina and exchanged clay s, endo (2b) is the major product. However, the exo cycloadduct (3b) i s preferably obtained for reactions catalyzed by silica gel and alumin a treated with TiCl4 and ZnCl2 supported on silica gel.